a. Field of the Invention
The present invention relates generally to low-density cellular concretes, and, more particularly, to the low-density cellular cement grout which provides the radiation barrier for use in the decommissioning and removal of reactor components and similar structures presenting a radioactive hazard.
b. Background Art
The decommissioning of nuclear power plants, such as nuclear electrical power generating stations and warship propulsion systems, typically involves a dismantling of the reactor systems at the plant site or vessel, and then transporting these to a designated disposal/storage area.
In conventional nuclear power systems, the main components are housed in one or more containment vessels; for example, separate containment vessels are commonly employed for the steam generator and the reactor core, and these components are connected by various pipe lines which are originally designed for the circulation of heating/cooling water and/or other fluids. In the typical dismantlement operation, each containment vessel is cut out of the system and removed individually, with the internal components remaining housed inside the vessel.
Typically, the dismantlement and transportation operations are fraught with a number of difficulties. Firstly, the residual radioactivity in the vessel and its internal components is usually quite high and presents a continuing hazard, both to workmen and during transportation; typically, the residual radiation includes both gamma ray radiation and neutron radiation. The radiation hazard normally exists throughout the system, but is typically most intense in the case of the reactor vessel which originally contained the reactor core; in many cases, the radiation hazard is so intense that even simple cutting/removal operation require rotation of a number of workers, each of which can be exposed to the radiation for only a very limited period of time.
A particular problem in this context is how to provide effective shielding for the "hot" components in reactor vessels for transportation and long-term storage, so that they do not present a virtually perpetual hazard to personnel in the area. The presently accepted practice calls for a very thick (e.g., 8-10") steel plate casing to be built around the component/vessel, in a manner resembling a shell. The thick layer of iron in the shell provides highly effective shielding, but the technique is vastly expensive in practice. Not only are there the costs ordinarily associating with erecting such a structure of what amounts to be thick armor plate, but the cost is greatly compounded by the necessity of having personnel work in a radioactively "hot" environment. For example, personnel involved in erecting and welding the shell generally remain on a station for a brief time before having to be rotated off of their job, and the same goes for inspectors. Moreover, because of the sensitive nature of the work, the construction work and welding is for the most part performed by skilled boiler makers, at much higher rates than ordinary welding personnel. As a result, the cost of constructing the armor shells can amount to many millions of dollars for each reactor vessel/component.
Although it has been known to construct reactor building walls of concrete with imbedded steel punchings for shielding purposes, this technique is not a viable option for encasing reactor components during removal. Moreover, such concrete mixes, which are ordinarily dumped between forms to construct the walls, are extremely coarse and lack fluidity, and are therefore incapable of being pumped into and filling the sometimes intricate voids around reactor components and containment vessels, or through the complex and often small-diameter piping and chambers which are commonly present in these components.
Accordingly, there exists a need for a method for shielding reactor vessels and other components which are being removed from decommissioned nuclear power plants, without requiring the construction of massive armor shielding. Furthermore, there is a need for such a method which can be performed on a fairly quick basis so as to minimize the high personnel costs which are ordinarily associated with this type of work. Still further, there is a need for such a method which employs a comparatively inexpensive shielding material, and a material which is sufficiently fluid that it can be installed in and around components having fairly complex contours and cavities. Still further, there is a need for such a material which will effectively reduce the emitted levels of both gamma and neutron radiation.